The description of references in this Section is not intended to constitute an admission that any patent, publication or other information referred to herein is “Prior Art” with respect to the Present Invention, unless specifically designated as such.
Hydrostatic transmission (“HST”) systems and transaxles are well known in the industry, and are more fully described in, e.g., U.S. Pat. No. 5,314,387, the contents of which are incorporated herein by reference in its entirety. In general, a typical HST system possesses, inter alia, a hydraulic pump and a hydraulic motor mounted in a housing. The hydraulic pump and hydraulic motor are hydraulically-linked through a generally-closed hydraulic circuit, and both consist of a rotatable body with a plurality of reciprocating pistons mounted therein. A hydraulic fluid, such as oil, is maintained in the generally-closed hydraulic circuit, and the HST generally has a sump, or reservoir, with which the generally-closed hydraulic circuit can exchange oil. In certain instances, this sump may be formed by the housing itself.
The hydraulic pump is usually driven by an external motive source, such as a pulley and belt system or a plurality of drive shafts connected to an internal combustion engine. The reciprocating pistons engage a moveable swash plate and, as the hydraulic pump is rotated by an input source driven by the engine, the reciprocating pistons engage the swash plate. Other HST designs may use a radial piston or ball piston pump and motor design, but the general operation is, in any event, similar, and the Present Invention is not limited to use with a specific design.
Movement of the reciprocating pistons creates movement of the hydraulic fluid from the hydraulic pump to the hydraulic motor, causing the rotation thereof. Motor pistons are engaged against a fixed plate, and rotation of the hydraulic motor drives an output shaft engaged thereto. This output shaft may be linked to mechanical gearing and output axles, which may be internal to the HST housing, as in an integrated hydrostatic transaxle (“IHT”), or external thereto.
Further, the system is fully reversible in a standard HST. This means that, when the swash plate against which the pump pistons move is moved, the rotational direction of the motor can be changed, such as in a forward or reverse direction. In addition, there is a “neutral” position where the pump pistons are not moved in an axial direction, so that rotation of the pump does not create any movement of the hydraulic fluid.
HZTs are also known in the industry. Generally, an HZT is utilized in connection with a vehicle to provide for the independent control of each of the drive wheels of a vehicle. By way of example, HZTs are described in U.S. Pat. Nos. 5,078,222 and 6,775,976, the contents of which are both incorporated herein by reference in their entireties. Generally speaking, the HZTs in the aforementioned references comprise two reduction drives powered by independent self-contained hydraulic power units. Additionally, Eaton has developed and marketed zero-turn transaxles, commonly known as Eaton Models 771 and 781. The Eaton Model 771 is an assembly with one pump and one motor where two Eaton Model 771 HZT assemblies, a right and a left, are required for zero-turn drive. The Eaton Model 781 consists of two units similar to the Eaton Model 771 joined together to make one assembly.
Each of the HZTs described above possesses the generally-closed hydraulic circuit discussed above. Thus, each individual HZT possesses two distinct sides, namely, a high pressure side in which oil is being pumped from the hydraulic pump to the hydraulic motor, and a low pressure, or vacuum, side, in which oil is being returned from the hydraulic motor to the hydraulic pump. When the swash plate angle is reversed, the flow out of the hydraulic pump reverses so that the high pressure side of the circuit “becomes” the vacuum side, and vice versa. This generally-closed hydraulic circuit can be formed as porting within the HZT housing, internal to a center section on which the hydraulic pump and hydraulic motor are rotatably mounted or in other ways known in the industry. Check valves may also be used to draw hydraulic fluid into the low pressure side to make up for fluid lost due to leakage, for example. Such check valves may be located so that they directly contact the porting or they may be located separate from the porting and connected through additional bores to the closed circuit. Additionally, an HZT may be provided with a means by which the closed loop may be opened to the sump or the high and low pressure side connected together. The means is commonly known as a “bypass” or a “bypass circuit.”
There is a need to provide a means to open, or bypass, this closed circuit in certain circumstances. This is necessary because when the vehicle is stopped, the oil in the closed circuit provides for hydraulic braking, making it difficult to manually move the vehicle. Mechanical bypass designs are known in the art and are described in, for example, U.S. Pat. No. 5,423,182, the contents of which are incorporated herein by reference in its entirety. Such designs generally achieve bypass by opening the closed hydraulic circuit to the sump by, e.g., opening check valves in the circuit or by opening a shunt between the high pressure and low pressure sides of the circuit. However, due to the nature of HZT configurations, the need exists for an improved bypass linkage assembly that can obtain bypass through the pull of only one handle.